A very detailed post at the Building Windows 8 blog about the graphics subsystem in Windows 8 - very interesting. One part stood out to me, though: "The Metro style design language is typographically rich and a number of Metro style experiences are focused on providing an excellent reading experience. DirectWrite enables great typographic quality, super-fast processing of font data for rendering, and provides industry-leading global text support. We've continued to improve text performance in Windows 8 by optimizing our default text rendering in Metro style apps to deliver better performance and efficiency, while maintaining typographic quality and global text support." All this still doesn't explain why text rendering on Metro (so not the classic desktop) had to be made as horrible as it is. Please, for the love of god, give us the option of turning ClearType's subpixel RGB optimisation back on for Metro applications. Fonts look horrible without it, which is kind of ironic given how font-heavy Metro is.

The entire display is being rotated 90! That is jarring enough to completely obscure any minor font rendering changes.

So one way the fonts look good and the other look less good? What is the point of rotating the display, as people will only ever use it in the better looking orientation?

Minor changes in fonts tend to be easily detected and I'd hypothesize that most people would prefer to get used to a single less good (but consistent) representation as opposed to two differing representations of the same text. It would be an interesting test to try.

How may hundreds of kilobytes would it take to have two font caches? Again the screen is being rotated 90 degrees, the entire display will have to be redrawn regardless.

In many cases the screen only has to be composited again upon rotation(textures scaled or moved around). But often text will have to be reflowed (in large documents), so it is as you say (redrawn anyway). Two font caches are slightly more inefficient though.